Glucagon and Related Peptides
Glucagon is a hormone that is released in response to low glucose levels and stimulates glucose production. Thus, it plays a role in counteracting insulin in blood glucose homeostasis (Unger and Orci, 1990, Glucagon in Diabetes Mellitus, 4th edition, Elsevier p. 104–120). Glucagon arises from the post-translational processing of a larger precursor molecule, proglucagon.
Proglucagon is produced in both the a-cells of the pancreas as well as in the enteroendocrine L-cells of the intestine. It is subject to differential processing in the different tissues in which it is expressed. For example, glucagon is selectively excised from the precursor in the pancreas while two smaller peptides, glucagon-like peptide-1 (GLP-1) and glucagon-like peptide-2 (GLP-2), are produced in the intestine. GLP-1 and GLP-2 consist of amino acid residues 78–107 and 126–158 of proglucagon respectively (Bell et al., 1983, Nature 304: 368–371; Buhl et al., 1988, J. Biol. Chem., 263:8621; Nishi and Steiner, 1990, Mol. Endocrinol. 4:1192–1198; Irwin and Wong, 1995, Mol. Endocrinol. 9:267–277).
Glucagon and GLP-1 have competing biological activities. GLP-1 stimulates insulin secretion, glucose uptake, and cAMP formation in response to the presence and absorption of nutrients in the gut, whereas glucagon increases glucose output by the liver, skeletal muscle tissue, and adipose tissue during periods of fasting (see, e.g., Mojsov, 1992, Int. J. Pep. Prot. Res. 40:333–343; Andreasen et al., 1994, Digestion 55:221–228). Specific GLP-1 receptors have been identified (Thorens, 1992, Proc. Natl. Acad. Sci. 89:8641–8645) which are distinct from the glucagon receptor (Jelinek et al., 1993, Science 259:1614–1616).
GLP-2 is 33 amino acid fragment of proglucagon. Various vertebrate forms (including human) of GLP-2 have been reported. GLP-2 has intestinotrophic activity (U.S. Pat. No. 5,834,428).
When administered exogenously, GLP-2 can produce a marked increase in the proliferation of small intestinal epithelium in mice, with no apparent side effects (Drucker et al., 1996, Proc. Natl. Acad. Sci. 93:7911–7916). Moreover, GLP-2 increases maximal transport rate of D-glucose across the intestinal basolateral membrane (Cheeseman and Tseng, 1996, Am. J. Phys. 271:G477–G482). GLP-2 may act via a G-protein-coupled receptor (Munroe et al., 1999, Proc. Natl. Acad. Sci. 96:1569–1573).
Disorders
Obesity is one of the most common medical disorders, affecting about 40% of the American population. Mortality from obesity in the United States is estimated at 300,000 to 400,000 per year. Although the etiology of obesity is not fully understood, obesity occurs when energy intake exceeds energy expenditure. Hypothalamic structures, which have complex interconnections with the limbic system and other brain structures, control appetite. In addition, the amount and distribution of a person's body fat may be genetically predetermined and influenced by hormones. Among the agents known to be involved in appetite control are leptin, GLP-1, GLP-2, and neuropeptide-Y.
Osteoporosis is the most common form of metabolic bone disease. It affects more than 25 million people in the United States and causes more than 1.3 million bone fractures each year, including approximately 500,000 spine, 250,000 hip and 240,000 wrist fractures. Hip fractures are the most serious consequence of osteoporosis, with 5–20% of patients dying within one year of the fracture and over 50% of survivors being incapacitated.
Osteoporosis is commonly observed in post-menopausal women, but it also occurs in elderly and young individuals. The disease is characterized by low bone mass and a deterioration of bone tissue, with a consequent increase in bone fragility and susceptibility to fracture. Although the etiology of osteoporosis is not known, its onset is associated with several factors such as increased age, decreased hormone level, and decreased calcium levels. Osteoporosis may occur in elderly men as androgen levels fall. Androgens play an important role in bone formation/maintenance and promote the synthesis of collagen, which provides a repository for the calcium and phosphorus. Osteoporosis may also be due to increased secretion of parathyroid hormone, which reduces bone formation and enhances bone absorption. Osteoporosis can also be caused by kidney degeneration, which reduces the activity of hydroxylase-activating vitamin D, decreasing intestinal calcium absorption, and precipitating the loss of bone matrix. Mobilization of nutrient stores in bone can be achieved by stimulating osteoclastic bone resorption. Likewise, resorptive activity can be reversed by increasing dietary availability of nutrients.
Dietary intake of calcium has been shown to regulate bone metabolism. Intake of oral glucose has recently been shown to decrease bone resorption, resulting in a fully expressed decrease within two hours following glucose administration (GB Patent Application No. 0007492.2). This response to glucose intake is independent of gender and age. A comparable effect was also demonstrated following protein administration (unpublished communication).
Bone-related disorders are characterized by bone loss resulting from an imbalance between bone resorption and bone formation. The potential for bone loss is directly related to the bone's normal rate of resorption and can amount to over 5% per year in humans immediately following menopause.
There are currently two main types of pharmaceutical treatment for osteoporosis, both aimed at reduction of bone resorption. The first involves the administration of an anti-resorptive compound. For example, estrogen has been used as an anti-resorptive agent to reduce fractures. However, estrogen fails to restore bone to levels of that in a skeleton of a young adult. Furthermore, long-term estrogen therapy has been implicated in a variety of disorders, including an increase in the risk of uterine cancer, endometrial cancer, and possibly breast cancer (Persson et al., 1997, “Hormone replacement therapy and the risk of breast cancer. Nested case-control study in a cohort of Swedish women attending mammography screening”, Int. J. Can. 72:758–761). For these reasons, many women avoid treatment of osteoporosis with estrogen.
A second type of pharmaceutical therapy for treating osteoporosis uses an agent that inhibits bone resorption as well as promotes bone formation and increases bone mass. These agents, such as alendronate, typically restore the amount of bone to that of an established premenopausal skeleton. However, alendronate administration can cause undesirable side effects, for example, or gastric ulceration (Graham et al., 1999, Aliment Pharmacol. Ther. 4:515–9).
The significant risks associated with the currently available pharmaceutical therapies (such as estrogen and alendronate) highlight the need to develop safer therapies for treating or preventing osteoporosis and other bone-related disorders. Therefore, there is a need for methods for treating or preventing a bone disorder, such as osteoporosis, that do not carry the aforementioned risks.